The ease of forming two-dimensional arrays is one advantage of a Vertical-Cavity Surface-Emitting Laser (VCSEL) over edge-emitting lasers. Such arrays are particularly useful in applications where each element of the array can be individually addressed, for example where it is desirable to produce a sequentially-scanning laser source. However, due to its low defect density, n-type substrates are preferred in constructing a VCSEL. With a conductive substrate, individual addressing each array element can only be achieved by electrically isolating the p-contact of each element from another element in the array, instead of in the more convenient row-column selection configuration. Furthermore, to minimize temperature rise of the individual elements, and thermal crosstalk between array elements from degrading VCSEL performance, in creating the assembly/packaging of a scanning laser source, it is preferred to mount the VCSEL die with the p-side down onto a sub-mount having high thermal conductivity.
To connect each of the isolated elements of a large array (e.g., a 16×16 array) to the laser driver circuitry with passive traces on the sub-mount, however, requires complex multi-layer construction of the sub-mount. Such complex submounts, even if feasible, impede the heat flow from the VCSEL. This effectively defeats the advantage offered by the p-down configuration. Furthermore, the configuration requires an individual driver for each of the array elements.
One solution to the above-noted issue is to use an active matrix row-column switch to simplify the routing required on the sub-mount, but such active matrix switches similarly interfere with the heat flow from the VCSEL. Even if the thermal degradation introduced by a multi-layer sub-mount and/or an active matrix switch chip can be managed, light must emit from the n-substrate side and, therefore, limits the VCSEL to operate at a long wavelength that is transparent to the substrate (e.g., for a GaAs-based VCSEL, the wavelength used must be longer than 940 nm). However, the n-doping of the substrate increases the free carrier absorption especially at longer wavelengths, which lower the efficiency of the VCSEL. Accordingly, a VCSEL array is desired that is directly row-column addressable while maintaining high thermal conductivity without restriction on the emission wavelength.